ROTATING ELECTRICAL MACHINES

Abstract: In a large rotating electrical machine having a rotor (1) comprising bar conductors (3) of copper located in slots (5) in a rotor core (7) and connected together at their ends to form a winding, the bar conductors are electrically insulated from the rotor core by a coating on the conductors of a ceramic based material. The coating serves to prevent sparking occurring between the conductors bars and the rotor core.

Description: ROTATING ELECTRICAL MACHINES This invention relates to rotating electrical machines. More particularly the invention relates to rotating electrical machines rated at powers in excess of 10 kilowatts, of the kind having a rotor comprising bar conductors of copper or copper-based material located in slots in a rotor core of magnetic material and connected together to form a winding. Such a machine is hereafter referred to as a rotating electric machine of the kind specified. Examples of such machines are induction motors wherein the rotor bar conductors are connected at their ends to form a cage winding and synchronous motors wherein the rotor conductors are connected to form a damper cage winding. In such machines the rotor bar conductors are usually uninsulated. However, in recent years it has become apparent that in some circumstances sparking can occur between the uninsulated conductor bars and the rotor core. Such sparking is unacceptable in machines required to operate in explosive atmospheres. It has been discovered that such sparking can be prevented by ensuring that the bar conductors are continually in perfect electrical contact with the rotor core. However, whilst such contact can be achieved in a new machine, it is impossible to guarantee that such contact is maintained throughout the service life of the machine, due for example to the different thermal expansions of the materials of the bar conductors and the rotor core. An alternative solution to the problem of sparking is to insulate the bar conductors electrically from the rotor core. However the selection of a satisfactory method of insulation has proved difficult. For example, wrapping the conductors in insulating tape or sheet has proved unsatisfactory because it is not possible to wrap the tape or sheet sufficiently tightly to achieve adequate tightness of the wrapped conductors in the rotor core slots to avoid fatigue failure of the bars. It is an object of the present invention to provide a rotating electric machine of the kind specified wherein this problem is alleviated. According to the present invention there is provided a rotating electrical machine of the kind specified wherein the rotor bar conductors are electrically insulated from the rotor core by a coating on the conductors of a ceramic based material. The coating suitably consists of an alumina ceramic based material and has a thickness of less than 500μm. Preferably the coating comprises a micro-porous ceramic based material, such a coating being suitably applied to the conductor bars using a plasma spraying process. Where the coating is of micro-porous material it is suitably impregnated with a synthetic resin material. Two rotating electrical machines in accordance with the invention will now be described, by way of example, with reference to the accompanying drawing which is a diagrammatic perspective view of part of the rotors of the machines. Referring to the drawing, the first machine to be described is an induction motor rated at 1500 kilowatts and having a cage rotor 1 comprising 50 copper rotor conductor bars 3 disposed in slots 5 around the periphery of the rotor core 7, each conductor 3 having an L-shaped cross-section of overall dimensions 10.04 by 45,74 millimetres and a web portion 9, i.e. a vertical limb portion, of dimensions 3.5 by 10 millimetres. The rotor core 7 is of sectional form and fabricated in conventional manner from laminations of magnetic material of thickness 0.5 millimetre. Prior to insertion in the rotor slots 5 the rotor conductor bars 3 are provided with an electrically insulating coating by plasma spraying them using an alumina ceramic powder in an inert gas in a plasma spray gun. Plasma spraying processes are in themselves well known, see for example, an article entitled "Plasma-sprayed Coatings" appearing on pages 78 to 83 of the September 1988 issue of Scientific American. A plasma spray gun typically comprising a water-cooled cone-shaped cathode inside a water-cooled cylindrical anode which extends beyond the cathode to form a nozzle. A stream of the inert gas, typically argon mixed with hydrogen, is passed through the space between anode and cathode where it ionizes and forms a plasma. The powder material is directed via a tube into the plasma jet which forms in the nozzle. The direct current supply connected between the anode sustains an arc in operation which in turn produces a plasma flame which jets out of the nozzle. The flame contains melted powder particles and propels them onto the target where they solidify to form a coating. After being formed by the plasma spraying process the coatings are impregnated with a synthetic resin material using any convenient known process, for example a vacuum impregnation process. The coated conductor bars 3 are inserted in the rotor core slots 5 and connected together to form a cage winding by brazing or welding at their ends to endrings 11 of copper or copper alloy. In the second machine, instead of forming the coatings using a plasma spraying process and impregnating, the coatings are formed by firing after dipping the conductor bars 3 into a liquid suspension of a suitable ceramic based material, for example a suspension of a glass ceramic powder in water. The use of a ceramic based material to form the insulating coatings on the conductor bars 3 provides very robust coatings which adhere strongly to the conductors and provide the degree of electrical insulation required, while at the same time giving the required level of heat transfer from the conductor bars 3 to the rotor core 7. Furthermore, the coatings are sufficiently robust to withstand the significant deformation of the ends of the conductor bars 3 which occurs during brazing or welding to the endrings 11. It will be appreciated that ceramic materials other than alumina and glass ceramics may be used for the conductor bar insulating coatings of machines in accordance with the invention. In general materials which have hitherto been used for purposes such as providing heat resistant and/or wear resistance coatings on components such as internal combustion engine parts, e.g. oxides based on zirconium, as described in the above mentioned article in "Scientific American", may be expected to be satisfactory.

Claims: CLAIMS 1. A rotating electrical machine rated at a power in excess of 10 kilowatts having a rotor comprising bar conductors of copper or copper based material located in slots (5) in a rotor core (7) of magnetic material and connected together to form a winding characterised in that said bar conductors (3) are electrically insulated from the rotor core (7) by a coating on the conductors (3) of a ceramic-based material. 2. A machine according to Claim 1 wherein the coating consists of an alumina ceramic based material. 3. A machine according to Claim 1 or Claim 2 wherein the coating has a thickness of less than 500μm. 4. A machine according to any one of the preceding claims wherein the coating comprises a micro-porous ceramic based material. 5. A machine according to Claim 4 wherein the coating is applied to the conductor using a plasma spraying process. 6. A machine according to Claim 4 or 5 wherein the coating is impregnated with a synthetic resin material. 7. A machine according to any one of Claims 1 to 3 wherein the coating is applied to the conductor bars (3) by firing after dipping in a liquid suspension of the ceramic based material. 8. A machine according to Claim 7 wherein the ceramic based material is a glass ceramic based material.

Inventors: CLARK PAUL EATON, REGAN ROGER HENRY, CLARK, PAUL, EATON, REGAN, ROGER, HENRY
Applicants: CLARK PAUL EATON, GEC ALSTHOM LTD, REGAN ROGER HENRY, CLARK, PAUL, EATON, GEC ALSTHOM LIMITED, REGAN, ROGER, HENRY
Application year: 1994
Main classification: H02K17
IPC codes: H02K3, H02K17